Something should happen those cities, because they're since it's release in Burning Crusade. It was never crowded there and probably never will be crowded there. I think we don't need them anymore as capitals. Here is another idea.
I have a theory, based on the the possible fact we will see Argus in a future expansion.
- Exodar will be restored as a flying spaceship and be back into business.
- Exodar flies to Argus to be the new neutral capital for both Alliance and Horde thanks to the acceptance and tolerance of prophet Velen.
- A smaller town will be left on the location of Exodar so the player can do the most important business there.
- Blood Elves magic powers hit a spaceship (like tempest keep etc..) full with Broken and Demons. Silvermoon will be partly destructed by a impact. The mana that has been shed in the impact opens several demon portals. The spaceship, filled with Burning Legion worshippers, was sent by Kil'Jeadan as his revenge for his lose at the Sunwell.
- Silvermoon guards has shutdown the city to save their citizens. While most of Silvermoon is full of demons and such the Walk of Elders is still in hands of the Blood Elves. They are heavily defending this part of the city. It will function as a 'town' to do the most important business there.
- Dire Maul is cleansed from it's threat. With help of Night Elf highbornes and some important members of Horde and Alliance a portal is opened there to travel towards Argus. The fortress of Dire Maul is well defended by both Horde and Alliance in case of losing the portal to a demon invasion from Argus.
Personally i think Dire Maul would be a great location to have a portal to a demon planet like Argus, because it is well fortified and can be well defended in case of a demon invasion from the portal. It fits lore mostly as well.
Self-created concept of the Argus portal in Dire Maul:
You're destroying it for the sake of getting rid of the BC starting zones. I think the capitals can stay and be the anchor/catalyst of finally updating those zones and adding them into their respective continent.